Laser sustained plasmas (LSPs) are used as broadband light sources in a variety of applications including wafer inspection and metrology. Some of the existing LSP illumination systems may experience difficulty in meeting specific power/radiance requirements for metrology and inspection applications. For example, LSP illumination systems may have limited brightness in broadband red and infrared (IR) spectral ranges. Brightness of LSP sources in the red and IR spectral ranges is limited by black-body emission limit. The black-body IR emission does not increase much with plasma temperature, leading to spectral radiance that struggles to exceed about 0.2 W/mm2/srad/nm in most practical cases, while the spectral radiance in ultraviolet (UV) ranges may exceed about 2 W/mm2/srad/nm. The plasma radiance is even lower in deeper IR ranges. Moreover, for most gases used in LSP, absorptivity in the IR range is higher than in the visible (VIS) to UV spectral ranges. When trying to achieve higher radiance in VIS to UV ranges, the gas density is increased to increase emissivity. Plasma absorptivity in the IR reaches the black-body limit very quickly (especially on the absorption lines) and colder plasma periphery absorbs emission from a hotter core, leading to an effective limit on the plasma radiance in the IR range corresponding to lower temperatures than the plasma core.
Some LSP illumination systems are also impeded by limited spectral width of the laser sources. When laser sources are used, their radiance can be very high and not limited by black body. However, the laser sources are narrow-band and cannot be used in applications where broadband is preferable. Similarly, only a few laser wavelengths are available in the VIS-UV-DUV spectral ranges.
Furthermore, imbalance of spectral radiance can occur in LSP illumination systems that employ combined illumination from LSP and laser sources. In some versions, light collection from the plasma can be designed to occur collinearly with the pump laser. In this case, the laser wavelengths that are transmitted through LSP are added to the collected spectrum to form a narrowband line corresponding to the spectral radiance contributed by the laser source. However, for the pump powers required to sustain the plasma and for typical plasma transmission of about 20% for most absorptive cases, the spectral radiance of the transmitted laser radiation exceeds plasma radiance by orders of magnitude. In order to make the plasma brighter, higher pump power is required and the balance of spectral brightness is never achieved.
There is a need for improved LSP technologies to generate broadband illumination that meets power/radiance requirements at all applicable spectral ranges.